segment: Correct stream_time calc for negative applied rate
Updated gst_segment_position_from_stream_time and gst_segment_to_stream_time to reflect correct calculations for the case when the applied rate is negative.
Pasting from design docs:
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Stream time is calculated using the buffer times and the preceding SEGMENT
event as follows:
stream_time = (B.timestamp - S.start) * ABS (S.applied_rate) + S.time
For negative rates, B.timestamp will go backwards from S.stop to S.start,
making the stream time go backwards.
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Therefore, the calculation for applied_rate < 0 should be:
stream_time = (S.stop - B.timestamp) * ABS (S.applied_rate) + S.time
and the reverse:
B.timestamp = S.stop - (stream_time - S.time) / ABS (S.applied_rate)
https://bugzilla.gnome.org/show_bug.cgi?id=756810